2 Answers
2

The episode is a 2 parter from Season 2 and near the end of the second part ("Cartman's Mom Is Still a Dirty Slut") in the hospital that Dr. Alphonse Mephesto was recovering in it's revealed that Liane is a Hermaphrodite and that the test indicated she was the father. A clue to this is how willingly Mr Garrison was to have sex with her given him being gay as indicated in how he showed up in the flash backs.

Following that Dr. Mephesto also mentions that since Liane is the father, she had to have gotten another woman pregnant at that dance who would be Eric's real mother. After another "Who is Eric's mother?" listing possible candidates (such as Sheila Broflovski which would really piss him off) Eric gives up on the idea.

Of course from the episodes I have seen it continues as if the episode never happened. I don't know if any other episode after that contradicted this personally however according to AJ's answer it is later contradicted in Season 14.

indeed, this answer was later retconned.
– CarmeisterMay 2 '18 at 10:05

@Carmeister does that make the retconned version the answer? Or more just like another suggestion until they may revisit the idea of who is Cartman's dad?
– BruceWayneMay 4 '18 at 0:13

I think this answer is good to have since it was the canonical answer (as far as anyone knew) for a long time. As for whether it will be changed again, it's always possible but I don't think there's any reason to suspect it will be.
– CarmeisterMay 4 '18 at 9:19